This page discusses the anatomy, evidence and limitations of the block in the setting of knee surgery. For a block "how to do it", see the saphenous block page.

The adductor canal block (ACB) is a compartmental block performed by depositing LA within the adductor canal - an aponeurosis covered tunnel in the middle third of the thigh. The adductor canal contains two main nerves providing sensory innervation to the knee – the saphenous nerve and the posterior division of the obturator nerve. Adductor canal block has been proposed as an advance on the existing gold standard following knee surgery, the femoral block, because Adductor canal block theoretically avoids quadriceps weakness. Quadriceps weakness is a significant limitation of femoral block 1 as it can potentially hinder mobilization and increases the fall risk. 2 Preservation of quadriceps strength after the adductor canal block has been recently confirmed in a randomized controlled trial involving volunteers. 3That said, the adductor canal block will not provide analgesia after knee surgery as good as that achieved by a more proximal femoral/obturator block (see below for reasons why).

Anatomy

The adductor canal is bordered by 3 muscles: anterolaterally the quadriceps muscle (specifically vastus medialis); posteriorly adductor magnus, and medially the sartorius muscle. It extends from the upper/anterior thigh femoral structures to the lower/medial thigh adductor hiatus – the distal opening in the adductor magnus muscle approx. 13 cm proximal to the knee. The femoral vessels leave the canal at the adductor hiatus, where they dive deeply, and this abrupt change in femoral artery depth is a useful indicator of the distal limit of the canal, and therefore the optimal level for LA placement. The surrounding muscles and in particular the artery, are useful landmarks to guide LA placement.

The canal contains the femoral artery, femoral vein, and nerves. Those nerves include the:

1. Posterior branch of the obturator nerve (entering the canal near the adductor hiatus): innervates the posterior knee joint capsule

b) Nerve to vastus medialis: innervates the quadriceps medialis muscle which in turn sends articular fibers to the knee

It is important to appreciate that the sensory innervation of the knee joint is not only from nerves passing through the adductor canal, but also from articular filaments arising from the nerves to the vastus lateralis and intermedius, which both arise from the femoral nerve proximal to the adductor canal, and in fact only just distal to the inguinal ligament. Therefore, a proximal femoral/obturator block will provide better analgesia for knee surgery than an ACB (with or without an obturator block). That said, the clinical relevance of these proximally arising femoral sensory nerves, in terms of mediating pain after knee surgery (compared to the adductor canal nerves), is at present unknown.

Cadaver studies have shown that 15 mL of LA is sufficient to fill the adductor canal although an MRI study supported the use of a 30 mL volume. 4

Evidence for its use for knee surgery

The adductor canal block has only been investigated in the setting of TKJR, not ACLR, and only two randomized trials have been conducted in the setting of TKJR. Jenstrup et al 5 randomized 71 patients undergoing TKJR under spinal anesthesia to receive 24 hours of intermittent bolus adductor canal block or placebo, starting in the PACU. Patients also received paracetamol, ibuprofen and PCA morphine. During the first 24 hours, total morphine consumption was reduced from 56 to 40 mg, as was pain during knee flexion (1.9 point reduction). Ambulation tests also favored the ropivacaine group.

Compared to placebo, when intermittent bolus adductor canal block was added to an anesthetic technique consisting of spinal anesthesia and single injection high volume local infiltration, pain at rest, pain on movement and ambulation were improved on the day of surgery, as was sleep quality on the first postoperative night. 6 The authors acknowledge that a significant proportion of catheters may have displaced from their original position, possibly accounting for the lack of benefit after 24 hours.

Limitations

As discussed above, the adductor canal block will not block the sensory nerves (innervating the knee) arising from the posterior division of the femoral nerve proximal to the adductor canal. Therefore, the ACB will not provide post knee surgery analgesia as effective as that produced by a combined femoral and obturator block. Although the adductor canal block has been shown to facilitate mobilization after TKJR (cf. no block), 5, 6 and femoral block has been shown to cause quadriceps weakness, 1 it is important to recognize that femoral block also enhances mobilization after TKJR. 7 This presumably relates to the improved analgesia and range of knee motion benefits outweighing the effect on quadriceps function. Ilfeld et al showed that compared to an overnight continuous femoral block (functionally a single shot block), a 4-day continuous femoral block improves worst pain on movement up until the end of the 4-day infusion period. The four day continuous femoral block also improved ambulation distance and time to discharge readiness. That study more accurately reflects current practice compared to the techniques included in a recent meta-analysis concluding that the continuous technique provided only minor benefits.

Given adductor canal block preserves quadriceps function better than femoral block, 3 the next issue to be answered in a randomized trial is whether the improved quadriceps strength achieved with the adductor canal block is negated by the consequent inferior analgesia, and the inevitable negative effect of this pain on mobilization. Regardless, should we be pursuing mobilization as a goal at the expense of adequate analgesia? That said, one positive from the adductor canal block is that it may encourage/engage a group of anesthesiologists to perform peripheral blocks for knee surgery when they previously avoided blocks in this patient group because of surgical resistance arising from the negative perception of quadriceps motor block (rightly or wrongly).

Logistical issues may also limit the adductor canal block. Being a compartment block, it is likely that intermittent boluses are required to block all nerves within the canal. This will necessitate either nurse administered boluses or a pump enabling relatively high bolus volumes. The site of catheter placement is not only at the level of the thigh tourniquet, but also close to the knee, which may increase the risk of catheter displacement. 6